Long Term Evolution, LTE, is the fourth-generation, 4G, mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In a UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a Node B, NB, in UMTS, and as an evolved Node B, eNodeB or eNodeB, in LTE. An RBS or access node is a general term for a radio network node capable of transmitting radio signals to a user equipment, UE, and receiving signals transmitted by a user equipment.
The “Mobile and Wireless Communications Enablers for the Twenty-twenty, 2020, Information Society”, METIS, is a project co-founded by the European Commission and is a consortium of 29 partners. The project objective is to respond to societal challenges for the year 2020 and beyond by laying the foundation for the next generation of the mobile and wireless communications system.
The frequencies being discussed for fifth generation, 5G, mobile communication systems in e.g. the METIS project, are much higher than what is commonly used for 3GPP accesses today—suggestions have been made for carrier frequencies >60 GHz.
Frequencies in the range 30 to 300 gigahertz are often referred to as Extremely high frequency, EHF, Radio waves in this frequency range have wavelengths from about ten to around one millimeter, giving it the name millimeter band or millimeter wave, sometimes abbreviated MMW or mmW.
Radio systems utilizing carrier frequencies in this range are typically less reliable than radio systems operating at lower frequencies, at least when it comes to mobile devices or devices in dynamic environments. This is mainly because EHF signals are associated with severe attenuation from, e.g., path loss, which is also why mmW systems tend to rely on beam forming to get decent link budgets. However, the available frequency spectrum is often plentiful at these high frequencies, enabling very high data rates; the latter being the typical motivation to look into mmW systems when very high user data rates is wanted.
Particularly beam forming in the uplink can be difficult for a mobile device, since just a small change in orientation can direct the beam in a very different direction. Further, traffic on a link is often asymmetric, i.e., downlink traffic tends to be larger in quantity than uplink traffic due to, e.g., downloading or streaming of large files.
Systems using mmW, such as IEEE standard 802.11ad, operate both uplink and downlink in the same frequency band. If the uplink is beam formed we then get the problems with robustness as described above. Further, for a pure downlink transmission like a file download or streaming, the only data in the uplink will be ACK/NAK messaging and possibly other types of feedback related to the transmissions, e.g., Channel Quality Indicator, CQI, messages or some other control messaging, as well as higher layer acknowledgements. This transmission will be of much lower rate than the downlink transmission which may be multi-Gbps or at least several 100 s of Mbps. Using uplink for mmW could therefore potentially be a waste of system resources since the link is dimensioned and optimized for very high data rates. Also, a small packet or data quantity transmitted via the uplink may cost much more energy than the corresponding transmission performed at lower frequencies.
To overcome these problems, published international patent application WO2013086164 proposes a method for establishment of a mmW link with uplink reporting via cellular (e.g. LTE) or mmW. Hence, it is proposed to rely on a cellular link, which is more reliable than the mmW, when setting up a mmW connection.
Furthermore, published international patent application WO2013086410 discloses a method where a user equipment has both mmW downlink and uplink capabilities, or only mmW downlink capabilities. The user equipments with only mmW downlink capabilities sends feedback information via a cellular system. The cellular system lower layers are used for mmW network control, connectivity and mobility management.
Hence, there is prior art suggesting using a comparably less volatile radio system for connection, for channel establishment, and/or for feedback. However, these disclosures do not go into detail about how such a solution may be implemented.
One problem that arises when using different radio access technologies for data transmission and feedback is the timing between downlink and uplink. WO2013086164 discusses that the mmW carrier is time aligned with LTE. However, it cannot be assumed that this is always the case.
Hence, there is still a need for further methods enabling reusing a different radio access technology, i.e., different from the radio access technology used in the downlink, as uplink feedback channel for example for mmW networks.